HomeMy WebLinkAboutGeoTech Report for BLD2007-00362 - BLD Engineering / Geo-tech Reports - 6/4/2007 MASON COUNTY
PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER
Shelton, Washington 98584 TQil B�
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DATE: June 4, 2007 A
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INTER-DEPARTMENTAL COMMUNICATIONS
TO: Chuck McCoy PARCEL # 12330-50-00082
FROM: John Sliva, Programs Engineer-PW BUILDING PERMIT NUMBER: BLD2007-00362
SUBJECT: Geotechnical Report Review NAME: Solberg Jim
Chuck:
The geotechnical report prepared for the residence at 30 NE Anchor Ct. at Belfairwas received and
reviewed by Public Works.
A review of the geotechnical report notes that the site had a structure previously and will be
redeveloped with a single-family residence. Steepest slopes on site are approximately 60 per cent.
Approximately 350 feet offsite to the south slopes do approach 100 per cent. The slope at the
proposed building location is approximately 2%. Depth to competent soil is approximately 10".
The report notes the proposed building location is stable relative to deep-seated instability and will
not be affected by the proposed structure. The proposed structure will not undermine adjacent
slopes. Slope stability was modeled in both static and dynamic conditions. The report sates a
building setback of 8 feet from the crest of the eastern and southeastern slopes is required. Due to
the septic tank location, no building setback will be required from the toe of the western slope.
The author writes that if their analysis and recommendations are followed, they do not anticipate
any on site or off site impact from the construction.
Adequate erosion and sediment control features need to be implemented during land disturbing
activities to protect neighboring properties and State waters from adverse stormwater runoff
impacts. The migration or release of silty water or mud from the applicant's property will be
considered a violation of County and State water quality protection regulations.
The report appears to satisfactorily address County requirements for Geotechnical Reporting.
Comments and recommendations contained in the report should be incorporated into the site
development plans and made conditions for permit issuance.
Please feel free to contact me at 724 if you have any questions regarding these comments, or if
you feel any features need further discussion or attention.
Si c rely, iv
John Sliva
rograms Engineer
GEOTECHNICAL TESTINGA 'UUiA041&"
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JIM SOLBERG 0-r`I 1 0 3 a
30 NE ANCHOR COURT
BELFAIR,WASHINGTON 98528
RE: GEOTECHNICAL REPORT
SITE INFORMATION: 30 NE ANCHOR COURT
BELFAIR,WASHINGTON 98528
GPS LOCATION: N47°27.0641 W122o 50.9051
LETTER DATE: 05/27/2008
REPORT DATE: 12/13/2005
Mr. Solberg:
As per our conversation earlier, this letter addresses the drainage concerns at the subject site. With our
recommendations for site drainage followed as per the original geotechnical report, we do not anticipate any
adverse conditions impacting site drainage at the subject site.
If you have any more questions,please call us at the number listed below.
0{ Wasb fa Respectfully Submitted,
y GEOTECHNICAL TESTING LABORATORY
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2439 �g 0 -Sed GeO Curtis Cushman
Engineering Geologist
CURTIS DEAN CUSHMAN
10011 Biomberg Street SW,Olympia,WA 98512
Phone#: (360)754-4612 Fax#:(360)754-4848
00
GEOTECHNICAL REPORT
30 ANCHOR COURT
BELFAIR, WASHINGTON
PREPARED FOR
JIM SOLBERG
BY
GEOTECHNicAL TESTING LABORATORY
OLYMPIA, WASHINGTON
APRIL 69 2007
GEOTECHNICAL TESTtrgG LABORATORY
CONTACT INFORMATION
PREPARER INFORMATION
GTL PROJECT NUMBER: 07-0152
CONTACT: LANCE LEVINE
ADDRESS: 10011 BLOMBERG STREET SOUTHWEST
OLYMPIA,WASHINGTON 98512
TELEPHONE: (360)754-4612
FACSIMILE: (360)754-4848
EMAIL ADDRESS: GEOTESTLAB@COMCAST.NET
CLIENT INFORMATION
CLIENT: JIM SOLBERG
TELEPHONE: (253)884-2351
CELLULAR TELEPHONE: (206)617-3475
BILLING ADDRESS: 1916 KP HIGHWAY NORTH
LAKEBAY,WASHINGTON 98349
SITE ADDRESS: 30 NE ANCHOR COURT
BELFAIR,WASHINGTON 98528
PARCEL: 123305000082
GPS LOCATION: N47,27.064'W 122o 50.9051
10011 Blomberg Street SW,Olympia,WA 98512 2
Phone#: (360) 754-4612 Fax#: (360)754-4848
GEO'TECHNICAL TESTING LABo to ToRy
SCOPE OF UNDERSTANDING
JIM SOLBERG
1916 KP HIGHWAY NORTH
LAKEBAY,WA 98349
RE: GEOTECHNICAL REPORT
30 NE ANCHOR COURT
BELFAIR,WA 98528
PARCEL 123305000082
N47°27.0641 W 122o 50.9051
Mr. Solberg:
As per your request,we have conducted a soils exploration, foundation evaluation, and slope stability analysis for
the above-mentioned parcel. The results of this investigation,together with our recommendations,are to be found
in the following report. We have provided three copies for your review and distribution.
Soil samples were submitted for laboratory testing from the project site. The data has been carefully analyzed to
determine soils bearing capacities, footing embedment depths and building setback distances. The results of the
exploration and analysis indicate that conventional spread and continuous wall footings appear to be the most
suitable type of foundation for the support of the proposed structure. Some variability was encountered in
comparing the soil profiles of the site. Net allowable soil pressures, embedment depth, and total expected
settlements have been presented for the site later in the report.
We are also a full service laboratory that can meet all your building, testing (compaction, asphalt, concrete), and
special inspection needs. We appreciate this opportunity to be of service to you and we look forward to working
with you in the future. If you have any questions concerning the above items,the procedures used,or if we can be
of any further assistance please call us at the phone number listed below.
Respectfully Submitted,
Was GEOTE NICAL TES NG LABORATORY
fa
Lance Levine,E.I.T.
Reviewed by,
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HAROLD PARKS Harold Parks,L.G.,L.E.G.
Senior Engineering Geologist
10011 Blomberg Street SW,Olympia,WA 98512 3
Phone#: (360)754-4612 Fax#: (360) 754-4848
GEOTECHNiCAL "TESTING LABORATORY
TABLE OF CONTENTS
CONTACTINFORMATION.....................................................................................................................................2
SCOPE OF UNDERSTANDING ...............................................................................................................................3
TABLEOF CONTENTS............................................................................................................................................4
INTRODUCTION.......................................................................................................................................................5
SITECONDITIONS...................................................................................................................................................6
SurfaceConditions..................................................................................................................................................6
SiteGeology............................................................................................................................................................7
SiteSoils.................................................................................................................................................................7
Subsurface Explorations.........................................................................................................................................8
SubsurfaceConditions............................................................................................................................................8
SlopeStability.........................................................................................................................................................8
CONCLUSIONS AND RECOMMENDATIONS......................................................................................................9
General....................................................................................................................................................................9
GEOLOGICALLY HAZARDOUS AREAS............................................................................................................. 10
Landslide Hazard Classification........................................................................................................................... 10
Seismic Hazard Classification .............................................................................................................................. 10
Erosion Hazard Classification .............................................................................................................................. 11
SlopeStability....................................................................................................................................................... 11
BuildingSetback................................................................................................................................................... 12
Seismic—Liquefaction Hazard............................................................................................................................. 14
ErosionControl..................................................................................................................................................... 15
EARTHWORK......................................................................................................................................................... 15
SitePreparation..................................................................................................................................................... 15
StructuralFill........................................................................................................................................................ 16
Suitability of Onsite Soils as Fill.......................................................................................................................... 16
Cutand Fill Slopes................................................................................................................................................ 17
FoundationSupport............................................................................................................................................... 17
FloorSlab Support................................................................................................................................................ 18
RetainingWalls..................................................................................................................................................... 18
RetainingWall Alternatives.................................................................................................................................. 19
SiteDrainage.........................................................................................................................................................20
SepticImpact.........................................................................................................................................................20
LIMITATIONS.........................................................................................................................................................20
Figure1 Vicinity Map...........................................................................................................................................21
rOlympia, 12 10011 Blomberg Street SW, A W 985 4
Phone#: (360) 754-4612 Fax#: (360) 754-4848
GEOTECHNICAL TESTING LABORATORY
INTRODUCTION
This report summarizes the results of our geotechnical consulting services for the proposed single-family
residence. The previous structure was removed in anticipation of construction. The site is located along the
southeast-facing hillside overlooking the Hood Canal in Mason County. The site is approximately 1.5 miles
northwest of Belfair, Washington. The location of the site is shown relative to the surrounding area on the
Vicinity Map,Figure 1.
4� 14
Our understanding of the project is based on our discussions with you and our explorations and review of the site.
We understand that the parcel is to be re-developed with a single-family residence. The site will be accessed by
an existing driveway from Anchor Court. In general, grading will consist of the excavation of the foundation and
footings. The approximate layout of the site is shown on the Site Plan,Figure 2.
The site slopes toward the east and southeast from the proposed building location. The steepest slope measured
onsite was approximately 60 percent. Therefore, Mason County requires that a geotechnical report be prepared in
accordance with the Critical Areas Ordinance.
The purpose of our services is to evaluate the surface and subsurface conditions at the site as a basis for providing
geotechnical recommendations and design criteria for the project and to satisfy the requirements of the Mason
County Critical Areas Ordinance. Geotechnical Testing Laboratory is therefore providing geologic and
hydrogeologic services for the project. Specifically, our scope of services for this project will include the
following:
1. Review the available geologic,hydrogeologic,and geotechnical data for the site area.
2. Conduct a geologic reconnaissance of the site area and surrounding vicinity.
3. Investigate shallow subsurface conditions at the site by observing the exposed soil and reviewing
published well logs.
4. Evaluate the landslide and erosion hazards at the site per the Mason County Critical Areas Ordinance
regulations.
5. Provide geotechnical recommendations for site grading including site preparation, subgrade preparation,
fill placement criteria (including hillside grading), temporary and permanent cut and fill slopes, drainage
and erosion control measures.
10011 Blomberg Street SW,Olympia,WA 98512 5
Phone#: (360) 754-4612 Fax#: (360)754-4848
SITE CONDITIONS
SURFACE CONDITIONS
The proposed building site is located in an area A
of heavy residential development in the Puget
Sound glacial upland overlooking the Hood
Canal. The site has an eastern and southeastern
exposure. We conducted a reconnaissance of ;•'
the site area on March 29, 2007. Site elevations
range from approximately 130 to 175 feet.
The building area of the site has vegetation
common to the Northwest. The vegetation f
includes fir, madrone, alder, and pine trees as
well as salal, Scot's broom, Oregon grape, Y `
huckleberry, bracken fern, sword fern, •=
blackberry,and grasses.
At the time of the site visit, we observed no a ' .•-�• ts.cM .,'
evidence of active surface erosion. No evidence '-• t''
.s -of deep-seated slope instability was observed
onsite. Minor raveling and sloughing was
observed along the southern driveway road cut. Offsite to the south are areas with slopes of 100 percent or
greater. Theses slopes are located approximately 350 feet from the proposed building location in the adjacent
gravel pit.
Surface water flow was not observed onsite. The general topography of the site area indicates that drainage flows
toward the southeast and east from the proposed building location.
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10011 Blomberg Street SW,Olympia, WA 98512 6
Phone#: (360) 754-4612 Fax#: (360)754-4848
GEOTECHNICAL TESTING LABORATORY
SITE GEOLOGY
The site is generally situated within the Puget Sound glacial upland. The existing topography, as well as the
surficial and shallow subsurface soils in the area, are the result of the most recent Vashon stade (stage) of the
Fraser glaciation that occurred between about 9,000 and 11,000 years ago, and weathering and erosion that has
occurred since. A description of the surficial soils is included in the"Site Soils"section of this report. In general,
the soils are composed of glacial material.
The Geologic Map of Washington—Northwest Quadrant(2002)has mapped the site geology as advance outwash
deposits(Qga)of continental glacial origin. The report reads:
Advance outwash — Glaciofluvial sand and gravel and lacustrine clay, silt, and sand deposited
during the advance of glaciers; sandy units commonly thick, well sorted, and fine grained, with
interlayered coarser sand, gravel, and cobbles; locally contains nonglacial sediments and
deposits mapped as transitional between glacial and nonglacial. Includes the Colvos and
Esperance Sand Members of the Vashon Drift and part of the Vashon Drift undivided
i r�',...ay...P. 1
4
SITE SOILS
The Soil Survey of Mason County, Washington, USDA Soil Conservation Service (1960)has mapped the site soils
as Everett gravelly sandy loam,0 to 5 percent slopes(Eg). The survey states:
This soil is on the steeper slopes of glacial moraines, sides of gullies, and terrace fronts. It is
closely associated with other Everett Soils and the Alderwood sandy loams. This soil is more
variable than Everett gravelly sandy loam, 5 to 15 percent slope. The depth of the substratum
ranges from 12 to 36 inches, and the amount of gravel in the surface soil and subsoil varies
greatly from place to place. Where the soil is in close association with the Alderwood soils, the
substratum, in places, is compact and weakly cemented.
10011 Blomberg Street SW, Olympia, WA 98512
Phone#: (360)754-4612 Fax#: (360)754-4848
GEOTECHNicAL TESTING LABORATORY
SUBSURFACE EXPLORATIONS
Subsurface conditions at the site were evaluated by observing the exposed building site soil and reviewing
available well logs. Groundwater was not encountered at the proposed building location and is beyond the scope
of this report(approximately 85 feet below ground surface). Depth to competent soil is approximately 10 inches
throughout the proposed building location.
SUBSURFACE CONDITIONS
In general, dense Everett gravelly loamy sand was observed in the undisturbed portions of the site. Vashon Stade
glacial material was observed below the Everett material. Groundwater was not observed or encountered.
Groundwater seepage was not observed onsite. Based on the site topography and the nature of the near surface
soil, seasonally perched groundwater conditions may not be expected during periods of extended wet weather.
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SLOPE STABILITY
The Relative Slope Stability of the Southern Hood Canal Area, Washington, (1977)describes the site area as Class
2. Class 2 is described as:
Areas believed to be stable under normal conditions, but may become unstable if disturbed by
man's activities, if slope is oversteepened by erosion, or if subjected to strong seismic shaking.
Slopes generally steeper than 15 percent, but may be less in some areas of weak geologic
materials. Includes areas underlain by: well-drained sand and gravel, mostly on valley sides
that lack known slope failures;glacial till with steep slopes; and bedrock.
Slopes approximately 60 percent were observed onsite. Offsite to the south, slopes approach vertical at the
abandoned gravel pit. Since slopes of 40 percent or greater with 10 feet or more of vertical relief occur onsite,
Mason County requires that a geotechnical report be completed according to the Critical Areas Ordinance.
The near-surface soils are in a dense to very dense condition except at the ground surface. The surficial soils are
generally in a medium dense condition.
In general,the undisturbed native soils of the site consist of a mixture of variable amounts of sand, silt,and gravel.
These soil materials are in a dense condition except where they have been disturbed by weathering activity. No
evidence of deep-seated landslide activity or significant erosion was observed onsite at the time of our
investigation.
10011 Blomberg Street SW, Olympia, WA 98512 g
Phone#: (360) 754-4612 Fax#: (360)754-4848
Weathering, erosion, and the resultant sloughing and shallow landsliding are natural processes that can affect
steep slope areas. Instability of this nature is typically confined to the upper weathered or disturbed zone, which
has been disturbed and has a lower strength. Only minor raveling and sloughing were observed along the southern
road cut.
Significant weathering typically occurs in the upper 2 to 3 feet and is the result of oxidation, root penetration,
wet/dry cycles, and freeze/thaw cycles. Erosion in steep slope areas such as this can be reduced by encouraging
vegetation and discouraging runoff from the steep slopes. Erosion control recommendations for the sloping areas
are provided in the"Erosion Control" section of this report.
.
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CONCLUSIONS AND RECOMMENDATIONS
GENERAL
Based on the results of our site reconnaissance, subsurface observations, and our experience in the area, it is our
opinion that the site is suitable for the proposed project. The proposed building location is stable relative to deep-
seated instability and will not be affected by the proposed structure. The proposed structure will not undermine
adjacent slopes. Proper drainage control measures will reduce or eliminate the potential for erosion in this area
and improve slope stability. The hazards of the landslide area can be overcome in such a manner as to prevent
harm to property and public health and safety,and the project will cause no significant environmental impact.
In general, the Everett soils observed at the site may be suitable for use as structural fill material. Saturated soil
conditions are not associated with these soils during or following extended periods of rainfall. However, to
reduce grading time and construction costs, we recommend that earthwork be undertaken during favorable
weather conditions.
Conventional construction equipment may be utilized for work at the site. Conventional spread footings may be
utilized at the site for support of the structure. We do recommend that roof and footing drains be installed for the
structure with conventional spread footings. A vapor barrier is recommended for all slab-on-grades.
Pertinent conclusions and geotechnical recommendations regarding the design and construction of the proposed
single-family residence are presented below.
10011 Blomberg Street SW,Olympia, WA 98512 9
Phone#: (360) 754-4612 Fax#: (360)754-4848
GEOTECHNIOAL TESTtNG LABORATORY
GEOLOGICALLY HAZARDOUS AREAS
LANDSLIDE HAZARD CLASSIFICATION
The Mason County Critical Areas Ordinance(17.01.100)defines a landslide hazard area as:
The following shall be classified as Landslide Hazard Areas:
a. Areas with any indications of earth movement such as debris slides, earthflows, slumps and rockfalls (see
figure F.100).
b.Areas with artificial oversteepened or unengineered slopes, i.e. cuts or fills.
c.Areas with slopes containing soft or potentially liquefiable soils.
d. Areas oversteepened or otherwise unstable as a result of stream incision, stream bank erosion, and
undercutting by wave action.
e. Slopes greater than 15% (8.5 degrees)and having the following:
i. Hillsides intersecting geologic contacts with a relatively permeable sediment overlying a relatively
impermeable sediment or bedrock(e.g. sand overlying clay); and
ii. Springs or groundwater seepage.
f Any area with a slope of forty percent or steeper and with a vertical relief of ten or more feet except areas
composed of consolidated rock. A slope is delineated by establishing its toe and top and measured by
averaging the inclination over at least ten feet of vertical relief.
SEISMIC HAZARD CLASSIFICATION
The Mason County Critical Areas Ordinance(17.01.102)defines a seismic hazard area as:
1.Areas susceptible to ground failure including the following:
a.Areas with Mapped geologic faults until proven inactive;
b. Deep road fills and areas of poorly compacted artificial fill;
c.Areas with artificially steepened slopes (i.e. old gravel pits);
d. Postglacial stream, lake or beach sediments;
e. River deltas;
f.Areas designated as potential Landslide Hazard Areas;
g. Bluff areas;and
h.Areas underlain by potentially liquefiable soils
2. The following criteria may be used as a guide by the County to indicate areas that have a higher likelihood of
meeting the classification criteria above:
a. Areas identified on the Coastal Zone Atlas of Washington, Volume 9, Mason County as Af, Qal, Qa2, Qvc,
Qls, Qos and Qp.
b.Areas identified on the Mason County Soil Survey Map as having slopes greater than 15 percent.
c. Faults identified on "Map Showing Known or Suspected Faults With Quaternary Displacement in the
Pacific Northwest", A.M. Rogers, T.J. Walsh, W.J. Kockelman and G.R. Priest, US Geologic Survey, 1996;
or described in "Active Faulting Investigations on the Canyon River Fault, Southern Olympic Range,
Washington", T.J. Walsh and KG. Neal, U.S. Geologic Survey, 1997.
d. Areas underlain by potentially liquefiable soils as shown "Liquefaction Susceptibility Map of Mason
County, Washington" by Stephen P. Palmer, Sammantha L. Magsino, James L. Poelstra, Eric L.
Bilderback Derek S. Folger, and Rebecca A. Niggemann, September 2004
This site does qualify as a seismic hazard area because the site is categorized as, "11 Areas designated as
potential Landslide Hazard Areas"
10011 Blomberg Street SW,Olympia,WA 98512 10
Phone#: (360)754-4612 Fax#: (360) 754-4848
GEO'I`ECHNICAL TESTING LABORATORY
EROSION HAZARD CLASSIFICATION
The purpose of the Erosion Hazard Section(17.01.104) is to identify areas that present potential dangers to public
health and safety, and to prevent the acceleration of natural geological hazards, and to neutralize the risk to the
property owner from development activities.
Areas in Mason County underlain by soils which are subject to severe erosion when
disturbed. Such soils include, but are not limited to, those for which potential for erosion
is identified in the Soil Survey of Mason County,USDA Soil Conservation Service, 1960,
or any subsequent revisions or additions to this source. These soils include, but are not
limited to, any occurrence of River Wash ("Ra') or Coastal Beaches ("Cg') and the
following when they occur on slopes I S%or steeper:
a.Alderwood gravelly sandy loam ("Ac"and"Ad')
b. Cloquallum silt loam ("Cd')
c. Harstine gravelly sandy loam("Hb')
d. Kitsap silt loam ("Kc')
The soils at the site are mapped as Everett gravelly loamy sand(Eg). This site does not meet the technical criteria
of an erosion hazard area.
41
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SLOPE STABILITY
Based on our field observations, explorations and our experience with the soil types encountered on the property,
we conclude that although slopes are approximately 60 percent, they are generally stable relative to deep-seated
failure in their present configuration. The proposed building location is in an area where the slope is
approximately 2 percent.
Excavation and back-filling will occur based on appropriate engineering and earthwork recommendations found in
the following"Earthwork" section. Grading in the building portion of the site should be conducted in accordance
with geotechnical recommendations provided herein.
As previously discussed, weathering, erosion, and the resultant surficial sloughing and landsliding are natural
processes that affect slope areas. Significant weathering typically occurs in the upper 2 to 3 feet and is the result
of oxidation, root penetration,wet/dry cycles and freeze/thaw cycles. Over excavation may be necessary to ensure
the removal of deleterious material.
These processes can be managed and the risk reduced through proper construction of the residence. Erosion
control recommendations in the slope and buffer areas are provided in the "Building Setback" and "Erosion
Control"sections of this report.
10011 Blomberg Street SW,Olympia, WA 98512 11
Phone#: (360)754-4612 Fax#: (360)754-4848
GEoTEcH cA L 'VESTING ]LABORATORY
iTORY
BUILDING SETBACK
A building setback (50 feet) from landslide hazard
areas is required unless evaluated and reduced by
an engineering geologist or a licensed professional
engineer. Based on our geotechnical evaluation of
the site and our experience in the area, a building
setback will be needed for this lot. The building
setback may be measured from the bottom of the
footing to the face of the steep slope in accordance setback
with the International Building Code (1805.3.1).
The following figure represents a shear angle for
the gravelly sandy loam. Shear angle and cohesion
are variables used to model the site.
Peak Shear Stress vs. Normal Stress
3000
38°
2500 — ----- -
w
ya 2000 —
N
N �
rn 1500 --
�`a
t I
Y
y 1000
a
Soo -- -- --
-+-1/4 ton
-1/2ton
-�1 ton
0 500 1000 1500 2000 2500 3000
Normal Stress(psf)
Slope stability was modeled using the GeoStudio 2004 program (version 6.20) in both static and dynamic
conditions (Ca = 0.3). Factors of safety were determined using Bishop's, Janbu, and the Morgenstern-Price
methods. The site was modeled using a monolithic layer of gravelly sandy loam. The material was determined to
have a unit weight of 132 pcf, cohesion of 200 psf, and a shear angle (�) of 38°. Under static conditions, the
slopes remained stable to deep-seated and shallow failure (F.S. = 2.00). Under dynamic loading, the 3328
computations demonstrated that the slope is not susceptible to surficial raveling or large deep-seated failure. The
following figure illustrates the moment factor of safety for slope"A" under the existing conditions for the lower
and upper slopes. The critical slip surface factor of safety is equal to 1.26 and 1.17. Mason County code requires
a factory of safety to be at least 1.1 at the proposed building location. This figure is the solution of greatest
concern and exhibits the need for a building setback of 8-feet from the crest of the eastern and southeastern slope.
Due to the septic tank location, no building setback will be required from the toe of the western slope. All
foundation elements shall be constructed on native material or engineered fill material. The proposed building
location meets the previous requirements.
10011 Blomberg Street SW,Olympia,WA 98512 12
Phone#: (360) 754-4612 Fax#: (360) 754-4848
GEOTECHNICAL TESTING LABORATORY
Solberg Site -- Slope A
1.
250 .
240 •
230 !
220
210 l .•. .
200 . . .
C 190 •. .
0 180
170 . .•.•.•.
w 160 Description:Gravelly Sandy Loam ,
150 Wt:132 �.
140 Cohesion:200
130 Phi:38
120
110
0 25 50 75 100 125 150 175 200 225 250 275 300
Distance(ft)
Solberg Site -- Slope A •• •' .•
250
240
230
220
210
200
r 190
2 180
iv 170
w 160 Description: Gravelly Sandy Loam
150 Wt: 132
140 Cohesion: 200
130 Phi: 38
120
110
0 25 5C 100 125 150 175 200 225 250 275 300
Distance (ft)
10011 Blomberg Street SW, Olympia, WA 98512 13
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GE®TECHNICAL TESTING LABORATORY
As previously discussed, weathering, erosion and the resultant surficial sloughing and shallow landsliding are
natural processes that affect slope areas. Minor surficial raveling and sloughing was observed along the southern
road cut. To manage and reduce the potential for these natural processes,we recommend the following:
➢ No drainage of concentrated surface water or significant sheet flow onto the sloped areas.
➢ No filling within the setback zone unless retained by retaining walls or constructed as an engineered fill.
SEISMIC—LIQUEFACTION HAZARD
According to the Seismic Zone Map of the United States contained in the 2003 International Building Code(IBC),
the project site is located where the maximum spectral response acceleration is 45 percent of gravity(g).
The Liquefaction Susceptibility Map of Mason County, Washington by Palmer, Magsino, Poelstra, Bilderback,
Folger,and Niggemann(September 2004)maps the site area as having a very low to low liquefaction potential.
The Site Class Map of Mason County, Washington by Palmer, Magsino, Bilderback, Poelstra, Folger, and
Niggemann (September 2004) maps the site area as site class C to D. Site class C is a very stiff soil or soft rock
and site class D is a stiff soil.
Based on the subsurface conditions observed at the site,we interpret the site conditions to correspond to a seismic
Soil Profile Type D, for Stiff Soil, as defined by Table 1615.1.1 (IBC). This is based on probing with a '/z-inch
diameter steel probe rod. The shallow soil conditions were assumed to be representative for the site conditions
beyond the depths explored.
Based on our review of the subsurface conditions, we conclude that the site soils are only mildly susceptible to
liquefaction. The near-surface soils are generally in a dense condition and the static water table is located well
below the surface. Shaking of the already dense soil is not apt to produce a denser configuration and subsequently
excess pore water pressures are not likely to be produced.
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10011 Blomberg Street SW,Olympia, WA 98512 1.4
Phone#: 360 754-4612 Fax#: 360 754-4848
GEOTECHHICAL TESTING LABORATORY
EROSION CONTROL
It is our opinion that the potential erosion hazard of the site is not a limiting factor for the proposed development.
Removal of natural vegetation should be minimized and limited to the active construction areas. Yard
landscaping around the home is permissible, but understory growth on the slopes should be encouraged as much
as possible as a deterrent to erosion. Hazard trees located on steep slopes may be removed only if the stumps
remain to deter erosion.
Temporary and permanent erosion control measures should be implemented and maintained during construction
and/or as soon as practical thereafter to limit the additional influx of water to exposed areas and protect potential
receiving waters.
Erosion control measures should include, but not be limited to, silt fences, berms, and swales with ground
cover/protection in exposed areas. A typical silt fence detail is included on Figure 2. Any re-contouring of the
site will create a need for erosion control measures as listed above.
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EARTHWORK
SITE PREPARATION
All areas to be excavated should be cleared of deleterious matter including any existing structures, debris, duff,
and vegetation. Based on our observations, we estimate that stripping on the order of 8 to 16 inches will be
necessary to remove the root zone and surficial soils containing organics. Areas with deeper, unsuitable organics
should be expected in the vicinity of depressions or heavy vegetation. Stripping depths of up to 3 feet may occur
in these areas. These materials may be stockpiled and later used for erosion control and landscaping. Materials
that cannot be used for landscaping or erosion control should be removed from the project site. No foundation
elements shall be constructed on fill material.
Where placement of fill material is required, the exposed subgrade areas should be proof-rolled to a firm and
unyielding surface prior to placement of any fill. We recommend that trees be removed with the roots, unless
located on a slope. Excavations for tree stump removal in any building area should be backfilled with structural
fill,compacted to the density requirements described in the"Structural Fill"section of this report.
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If structural fill is needed, we recommend that a member of our staff evaluate the exposed subgrade conditions
after removal of vegetation and topsoil stripping is completed.
Any soft, loose or otherwise unsuitable areas delineated during foundation preparation or probing should be
compacted, if practical, or over-excavated and replaced with structural fill, based on the recommendations of our
report.
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STRUCTURAL FILL
All fill material should be placed as structural fill. The structural fill should be placed in horizontal lifts of
appropriate thickness to allow adequate and uniform compaction of each lift. Fill should be compacted to at least
90 percent of MDD (maximum dry density as determined in accordance with ASTM D-1557) to within 2 feet of
subgrade and 95 percent MDD in the upper 2 feet.
The appropriate lift thickness will depend on the fill characteristics and compaction equipment used. We
recommend that the appropriate lift thickness be evaluated by our field representative during construction.
The suitability of material for use as structural fill will depend on the gradation and moisture content of the soil.
As the amount of fines (material passing No. 200 sieve) increases, soil becomes increasingly sensitive to small
changes in moisture content and adequate compaction becomes more difficult to achieve. During wet weather,we
recommend the use of well-graded sand and gravel with less than 9 percent(by weight)passing the No. 200 sieve
based on that fraction passing the 3/4-inch sieve.
If prolonged dry weather prevails during the earthwork and foundation installation phase of construction, a
somewhat higher(up to 10 percent)fines content will be acceptable.
Material placed for structural fill should be free of debris, organic matter,trash, and cobbles greater than 6 inches
in diameter. The moisture content of the fill material should be adjusted as necessary for proper compaction.
SUITABILITY OF ONSITE SOILS AS FILL
Onsite soils may be considered for use as structural fill if industry standards are satisfied. In general, the native
soils(sand, silt, and gravel)encountered on the site must have less than 10 percent fines(material passing the US
No. 200 sieve)to be suitable for use as structural fill.
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GEOTECHNicAL TESTING LABORATORY
► - Minor Fill Oftit�
CUT AND FILL SLOPES
All job site safety issues and precautions are the responsibility of the contractor providing services and/or work.
The following cut/fill slope guidelines are provided for planning purposes.
Temporary cut slopes will likely be necessary during grading operations. As a general guide,temporary slopes of
1.5 to 1 (horizontal to vertical) or flatter may be used for temporary cuts in the upper 3 to 4 feet of the glacially
consolidated soils that are weathered to a loose/medium-dense condition. Temporary slopes of 1 to 1 or flatter
may be used in the unweathered dense to very dense sands and gravel.
These guidelines assume that all surface loads are kept at a minimum distance of at least one half the depth of the
cut away from the top of the slope and that significant seepage is not present on the slope face. Flatter cut slopes
will be necessary where significant raveling or seepage occurs.
Surface drainage should be directed away from all slope faces. All slopes should be seeded as soon as practical to
facilitate the development of a protective vegetative cover or otherwise protected.
FOUNDATION SUPPORT
Where foundation elements are located near slopes between 5 and 30 percent, the footings should be located a
minimum of 2 times the footing width from the slope face (horizontally), and founded in medium dense or denser
native soils or properly prepared structural fill.
We recommend a minimum width for isolated and continuous wall footings to meet IBC 2003. Footings founded
as described above can be designed using an allowable soil bearing capacity of 2,000 psf(pounds per square foot)
for combined dead and long-term live loads in areas of medium dense to dense soils.
The weight of the footing and any overlying backfill may be neglected. The allowable bearing value may be
increased by one-third for transient loads such as those induced by seismic events or wind loads.
Lateral loads may be resisted by friction on the bases of footings and floor slabs and as passive pressure on the
sides of footings. We recommend that an allowable coefficient of friction of 0.40 be used to calculate friction
between the concrete and the underlying soil. Active pressure may be determined using an allowable equivalent
fluid density of 150 pcf(pounds per cubic foot).
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We estimate that settlements of footings designed and constructed as recommended will be less than 1 inch, for
the anticipated load conditions, with differential settlements between comparably loaded footings of '/2 inch or
less.
Most of the settlements should occur essentially as loads are being applied. However, disturbance of the
foundation subgrade during construction could result in larger settlements than predicted.
FLOOR SLAB SUPPORT
Slabs-on-grade should be supported on medium dense or dense native soils or on structural fill prepared as
described in the"Structural Fill" section of this report. We recommend that floor slabs be directly underlain by a
minimum 6-inch thickness of coarse sand and/or gravel containing less than 5 percent fines (by weight). The
drainage material should be placed and compacted to an unyielding condition.
A synthetic vapor barrier may be used for the control of moisture migration through the slab, particularly where
adhesives are used to anchor carpet or tile to the slab. A thin layer of sand may be placed over the vapor barrier
and immediately below the slab to protect the liner during steel and/or concrete placement. The lack of a vapor
barrier could result in wet spots on the slab,particularly in storage areas.
RETAINING WALLS
Retaining walls may be utilized on the sloping portion of the site to retain fill material. The lateral pressures
acting on the subgrade and retaining walls will depend upon the nature and density of the soil behind the wall. It
is also dependent upon the presence or absence of hydrostatic pressure. If the adjacent exterior wall space is
backfilled with clean granular, well-drained soil (washed rock), the design active pressure may be determined
using an active pressure coefficient equal to 0.25 (Ka= 0.25). This design value assumes a level backslope and
drained conditions as described below.
Retaining walls located on or near the toe of a slope that extends up behind the wall should be designed for a
lateral pressure, which includes the surcharge effects of the steep slope in proximity to the wall. Although not
expected at this site,the following data is provided for planning purposes.
For an irregular or composite slope, the equivalent slope angle may be determined by extending a line upward
from the toe of the wall at an angle of 1 to 1 (Horizontal to Vertical)to a point where the line intersects the ground
surface. The surcharge effects may be modeled by increasing the equivalent fluid pressure for flat ground by the
percentage given in the following table:
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SLOPE INCLINATION:EQUIVALENT FLUID PRESSURE
Slope Angle Percent Increase Equivalent Fluid Pressure
Horizontal 0% 35 pcf
3H:1 V 25% 44 pcf
2H:1 V 50% 53 pcf
I H:l V 75% 61 pcf
If the walls are greater than 4 feet in height, exclusive of the footing, additional design considerations should be
applied.
Positive drainage, which controls the development of hydrostatic pressure,can be accomplished by placing a zone
of coarse sand and gravel behind the walls. The granular drainage material should contain less than 5 percent
fines. The drainage zone should extend horizontally at least 18 inches from the back of the wall. The drainage
zone should also extend from the base of the wall to within 1 foot of the top of the wall. The drainage zone
should be compacted to approximately 90 percent of the MDD. Over compaction should be avoided as this can
lead to excessive lateral pressures.
A perforated PVC pipe with a minimum diameter of 4 inches should be placed in the drainage zone along the base
of the wall to direct accumulated water to an appropriate discharge location.
We recommend that a non-woven geotextile filter fabric be placed between the drainage material and the
remaining wall backfill to reduce silt migration into the drainage zone. The infiltration of silt into the drainage
zone can,with time,reduce the permeability of the granular material.
The filter fabric should be placed in such a way that it fully separates the drainage material and the backfill, and
should be extended over the top of the drainage zone.
Lateral loads may be resisted by friction on the bases of footings and as passive pressure on the sides of footings
and the buried portions of the wall. We recommend that an allowable coefficient of friction of 0.40 be used to
calculate friction between the concrete and the underlying soil.
RETAINING WALL ALTERNATIVES
Typically, block wall systems are more cost effective for long-term walls than the other options. Specific design
criteria for these options can be provided at your request by the block manufacturers.
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10011 Blomberg Street SW,Olympia,WA 98512 19
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GEOTECHNICAL UST[NO LABORATORY
SITE DRAINAGE
All ground surfaces, pavements and sidewalks should be sloped away from the residence and associated
structures. Surface water runoff should be controlled by a system of curbs, berms, drainage swales, and/or catch
basins and tight-lined into the appropriate drainage facilities. We recommend that conventional roof drains be
installed. Footing drains shall be installed for the single-family residence. The roof drain should not be
connected to the footing drain. For footing drains,the drain invert should be below the bottom of the footing.
We recommend that the collected stormwater runoff be directed into the appropriate drainage facilities by tight-
line. Drainage control measures are included on Figure 3. Onsite irrigation to lawn areas should be closely
monitored. We do not expect any adverse affects on the recharge condition of the groundwater system.
SEPTIC IMPACT
The existing septic drainfield will be located in the western portion of the site. The proposed drainfield location
setback will be greater than 40 feet from the top of any slope. We conclude the slope stability of the site will not
be adversely impacted by the existing septic drainfield.
LIMITATIONS
We have prepared this report for the use of Jim Solberg and members of his design team,to use in the design of a
portion of this project. The data used in preparing this report, and this report, should be provided to prospective
contractors for their bidding or estimating purposes only. Our report,conclusions and interpretations are based on
data from others and our site reconnaissance, and should not be construed as a warranty of the subsurface
conditions. This report is quantified as a micro-study and not a macro-study. Geotechnical Testing Laboratory
and its personnel cannot be responsible for unforeseen and widespread geologic events (such as earthquakes,
large-scale faulting, and mass wasting)beyond the scope of this project.
Variations in subsurface conditions are possible and may occur with time. A contingency for unanticipated
conditions should be included in the budget and schedule. Sufficient consultation should be made with our firm
during construction to confirm that the conditions encountered are consistent with those indicated by the
recommendations and for design changes should the conditions revealed during the work differ from those
anticipated,and to evaluate whether earthwork and foundation installation activities comply with contract plans.
I
If our analysis and recommendations are followed, we do not anticipate any on site or off site impact from the
construction. It is our conclusion that potential landslide hazards from the landslide area can be overcome so as
not to cause harm to property,public health and safety,or the environment.
The scope of our services does not include services related to environmental remediation and construction safety
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precautions. Our recommendations are not intended to direct the contractor's methods, techniques, sequences or
procedures,except as specifically described in our report for consideration in design.
If there are any changes in the loads, grades, locations, configurations or types of facilities to be constructed, the
conclusions and recommendations presented in this report may not be fully applicable. If such changes are made,
we should be given the opportunity to review our recommendations and provide written modifications or
verifications as appropriate. Recent chan es in the Mason County Code require our firm to revisit the site after
g ry (
six months)to ensure the conditions are in agreement with our original report.
1 11 Blomberg Street W 1 00 be g St e S , Olympia, WA 98512 20
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5
1/2 INCH MINIMUM DIAMETER STEEL ROD
(STRAP)CLAMPED SECURELY TO PIPE
CORRUGATED TIGHTLINE 4 INCH
MINIMUM,6 INCH SUGGESTED
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TIGHTLINE ANCHORED WITH TWO,
3 FOOT REBAR LENGTHS OR BOLTS.
JLFUARE END SECTIONQUARRY SPALL OR ENERGY �;;
DISPERSION DEVICE '<"`'';> . _. ;',;:.� ,ut,=:� .a?•:r: �:-....
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GRASS—LINED SWALE SHOULD BE A
MINIMUM ONE FOOT WIDE AT THE
BOTTOM AND ONE FOOT DEEP WITH
A MAXIMUM SLOPE OF 5 PERCENT.
MINIMUM 4 FEET
LEVEL SECTION
GEOTEXTILE FABRIC
Geotechnical Testing Laboratory
Geotechnical Services 10011 Blomberg st.SW
Olympia,WA 98512 FIGURE 3
Q/�QC Services Olympia,
(360)754-4612
Testing Services Fax:(360)7544848 Not to scale DRAINAGE DETAILS